1. The Field of the Invention
The present invention relates to vertical cavity surface emitting lasers. More particularly, the present invention relates to vertical cavity surface emitting lasers that generate high power in a single mode using photonic crystals.
2. Related Art
One of the light sources used in optical communication systems is a vertical cavity surface emitting laser (“VCSEL”). VCSELs are popular in part because they can be manufactured in large quantities due to their relatively small size and can be tested in wafer form. VCSELs typically have low threshold currents and can be modulated at high speeds. VCSELs also couple well to optical fibers.
In a VCSEL, the light or optical signal being amplified resonates in a direction that is perpendicular to the pn-junction. The cavity or active region of a VCSEL is thus relatively short and a photon has a small chance of stimulating the emission of an additional photon with a single pass through the active region. To increase the likelihood of stimulating the emission of photons, VCSELs require highly efficient mirror systems such that a photon can make multiple passes through the active region. The reflectivity requirement of VCSELs cannot easily be achieved with metallic mirrors.
VCSELs thus employ Distributed Bragg Reflector (DBR) layers as mirrors. The semiconductor materials or dielectric materials used in DBR layers are grown or formed such that each layer has a refractive index that is different from the refractive index of adjoining layers. The junctions between the DBR layers that are grown in this fashion cause light to be reflected. The amount of light reflected, however, by a single junction is relatively small and is often dependent on the variance between the relative refractive indices of the adjoining materials. For this reason, a relatively large number of DBR layers are formed in a VCSEL in order to achieve high reflectivity. VCSELs, for example, often have on the order of 50 to 100 DBR layers in order to achieve sufficient reflectivity.
Even though forming a large number of DBR layers can be a difficult task, VCSELs are attractive for various low power and/or multi-transverse mode applications because they are easy to test, have a good beam profile and typically have low power consumption. These attributes also make VCSELs unattractive for other applications, such as those that require more power in a single mode. For example, high power pump lasers are required for erbium doped fiber amplifiers. The power supplied by the pump lasers should be delivered in a single mode in order to avoid excessive noise. Current VCSELs are not suitable for this application because single mode VCSELs do not produce sufficient power. Those VCSELs that do produce sufficient power introduce excessive noise because more than one mode is generated. As a result, higher cost edge emitting lasers are often used for these applications.
A single mode high power VCSEL is difficult to produce for various reasons. The wavelengths reflected by the DBR mirrors depend on the composition and thickness of the DBR layers. The resonance wavelengths of the cavity depend on the composition and thickness of the cavity and of the DBR layers. Growing VCSEL wafers is difficult because the thickness, composition, and doping requirements must all be monitored at the same time for, potentially, hundreds of layers. Even when the growth and fabrication issues are overcome, high power VCSELs generate light output with high order transverse modes. A single mode high power VCSEL would overcome these and other problems.